The other issue with smooth rocks is they are hard to lock in place just ask anyone who uses them in their African ciclid tanks, they are a disaster waiting to happen as the fish dig
For helping to hold the smooth rocks in place I used a 2 part putty type epoxy from fuvel. It is also used for building salt reefs, and for anchoring corals to the rock. Took a bit to figure out how to work with it, but it works great for holding the rocks. I used it for the rocks in my crayfish tank, as well to make the pool for my salamander. The only suggestion is to look for one that dries black.